Complete mitochondrial genome of 5,000-year-old mummy yields surprise

Researchers have revealed the complete mitochondrial genome of one of the world's most celebrated mummies, known as the Tyrolean Iceman or Ötzi. The sequence represents the oldest complete DNA sequence of modern humans' mitochondria, according to the report published online on October 30th in Current Biology, a Cell Press publication. Mitochondria are subcellular organelles that generate all of the body's energy and house their own DNA, which is passed down from mother to child each generation. Mitochondrial DNA thus offers a window into our evolutionary past.

"Through the analysis of a complete mitochondrial genome in a particularly well-preserved human, we have obtained evidence of a significant genetic difference between present-day Europeans and a representative prehistoric human—despite the fact that the Iceman is not so old—just about 5,000 years," said Franco Rollo of the University of Camerino in Italy.

The Tyrolean Iceman witnessed the Neolithic-Copper Age transition in Central Europe more than 5,000 years ago. His mummified corpse was recovered from an Alpine glacier on the Austro-Italian border in 1991. In 2000, scientists defrosted the Iceman's body for the first time and sampled DNA from his intestines.

Earlier study of the DNA showed that he belonged to the lineage, or "subhaplogroup," known as K1. About 8% of modern Europeans belong to the K haplogroup, meaning that they share a common ancestor, and that group is divided into two "subhaplogroups," K1 and K2. The K1 haplogroup, in turn, can be divided into three clusters.

In the new study, the researchers took advantage of advanced genome-sequencing technologies to shed more light on the Iceman's genetics. They sequenced his entire mitochondrial genome and compared that sequence to other published human mitochondrial DNA sequences to construct his evolutionary (or phylogenetic) family tree.

"The surprise came when we found that the lineage of the Iceman did not fit any of the three known K1 clusters," Rollo said. His team has informally named the newly discovered branch on the human family tree "Ötzi's branch."

"This doesn't simply mean that Ötzi had some 'personal' mutations making him different from the others but that, in the past, there was a group—a branch of the phylogenetic tree—of men and women sharing the same mitochondrial DNA," Rollo said. "Apparently, this genetic group is no longer present. We don't know whether it is extinct or it has become extremely rare."

At least for the moment, he said, that means no one can claim to be "the issue of Ötzi."

Source: Cell Press